1. Field of the Invention
The present invention concerns an antenna arrangement for a magnetic resonance system, of the type having a number of antenna rods that are essentially arranged regularly around an antenna axis and proceed essentially axially relative to the antenna axis, wherein each antenna rod exhibits two rod ends and a rod center, the rod ends of each antenna rod being connected in an electrically-conductive manner with the rod ends of at least one of its immediately neighboring antenna rods via connection elements proceeding essentially tangentially relative to the antenna axis.
2. Description of the Prior Art
Antenna arrangements of the above-described type are generally known and are shown in FIG. 1 and FIG. 2.
FIG. 1 shows what is known as a birdcage resonator. The birdcage resonator comprises a number of antenna rods 1. The antenna rods 1 are regularly arranged around an antenna axis 2. They are even for the most part uniformly arranged around the antenna axis 2. They run essentially parallel to the antenna axis 2, advantageously even exactly parallel to the antenna axis 2.
The antenna axis 2 represents a preferred direction of a cylindrical coordinate system. The terms “axial”, “radial” and “tangential” are used herein as follows: “axial” designates a direction parallel to the antenna axis 2 and “radial” and “tangential” designate directions perpendicular to the antenna axis 2. The direction toward and away from the antenna axis 2 is designated as “radial”; the direction around the antenna axis 2 is designated as “tangential”.
According to FIG. 1, each antenna rod 1 exhibits two rod ends 3, 4 and a rod center 5. The rod center 5 is thereby equidistant from the rod ends 3, 4. Corresponding rod ends 3, 4 of the antenna rods 1 are connected with one another in an electrically-conductive manner via two closed ferrules 6, 7 or rings. Each antenna rod 1 is thus connected in an electrically-conductive manner with one of the ferrules 6, 7 per each of its rod ends 3, 4. The ferrules 6, 7 thus represent connection elements 6, 7 for electrically-conductive connection of the rod ends 3, 4, the connection elements 6, 7 proceeding essentially tangential to the antenna axis 2.
The antenna rods 1 are mirror-symmetrical with regard to a plane of symmetry due to the arrangement of the antenna rods 1 parallel to the antenna axis 2. The plane of symmetry is perpendicular to the antenna axis 2 and contains the rod centers 5.
With regard to each antenna rod 1, its rod center 5 defines with the antenna axis 2 a radially proceeding reference direction. Each point of the same antenna rod 1 defines with the antenna axis 2 a radial direction. Due to the circumstance that the antenna rods 1 of the antenna arrangement from FIG. 1 run parallel to the antenna axis 2, all radial directions thus are naturally identical with the reference direction. Each radial direction therefore forms with the reference direction an angle of 0° that remains constantly zero over the entire antenna rod 1.
The representation according to FIG. 2 essentially corresponds to that of FIG. 1. In contrast to the representation from FIG. 1, the connection elements 6, 7 of FIG. 2 are not fashioned as closed ferrules but rather as ferrule segments 6, 7. Precisely two rod ends 3, 4 are respectively connected with one another in an electrically-conductive manner via said ferrule segments 6, 7. As a result, the antenna rods 1 and the connection elements 6, 7 form a number of individual antennas 10 of an antenna array. The remainder the design of the antenna arrangement corresponds to that of FIG. 1.
Magnetic resonance excitation occurs in an examination volume (transmission operation of the antenna arrangement) and magnetic resonance signals can be received from the examination volume (acquisition operation of the antenna arrangement) by means of the generally known antenna arrangements described in the preceding. The examination volume is thereby essentially cylindrical, and the dimensions of the examination volume essentially correspond with the dimensions of the antenna arrangement.
It is also known (for example from WO-A-03/008988) to use an antenna arrangement that represents a modified form of the antenna arrangement of FIG. 1. In this antenna arrangement the antenna rods 1 are directed around the antenna axis 2 like a helix. This antenna arrangement can be imagined most simply as if the ferrules 6, 7 were skewed counter to one another in the antenna arrangement of FIG. 1. In this case the antenna rods 1 are naturally no longer mirror-symmetrical to the plane of symmetry. Rather, in this case each radial direction of a point (insofar as the point is not identical with the rod center 5) forms, with the reference direction, a tangential angle. For each antenna rod 1 the tangential angle is a non-constant function of the axial separation of the respective point from the rod center 5. As a rule the tangential angles increase linearly from the rod center 5 of each antenna rod 1 toward the rod ends 3, 4 of this antenna rod 1 with increasing axial spacing of the points from the rod center 5.
The excitation signals generated by the antenna arrangements of the prior art and the sensitivity of the antenna arrangements of the prior art to magnetic resonance signals is already quite good. Theoretical calculations, however, show that the antenna arrangements of the prior art are not yet optimal.